Reland [Syntax] Add minimal TableGen for syntax nodes. NFC

This reverts commit 09c6259.
(Fixed side-effecting code being buried in an assert)

Author: sam-mccall

clang/include/clang/CMakeLists.txt

@@ -5,3 +5,4 @@ add_subdirectory(Parse)
 add_subdirectory(Sema)
 add_subdirectory(Serialization)
 add_subdirectory(StaticAnalyzer/Checkers)
+add_subdirectory(Tooling/Syntax)

clang/include/clang/Tooling/Syntax/CMakeLists.txt

@@ -0,0 +1,4 @@
+clang_tablegen(Nodes.inc -gen-clang-syntax-node-list
+  SOURCE Nodes.td
+  TARGET ClangSyntaxNodeList)
+